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chromium/third_party/blink/web_tests/external/wpt/dom/events/mouse-event-retarget.html 

<!DOCTYPE html>
<html>
<title>Script created MouseEvent properly retargets and adjusts offsetX</title>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>

<style>
body {
  margin: 8px;
  padding: 0;
}
</style>

<div id="target">Hello</div>

<script>
async_test(t => {
  target.addEventListener('click', ev => {
    t.step(() => assert_equals(ev.offsetX, 42));
    t.done();
  });

  const ev = new MouseEvent('click', { clientX: 50 });
  target.dispatchEvent(ev);
}, "offsetX is correctly adjusted");
</script>